This invention pertains to novel high selectivity microporous aluminophosphate (AlPO4) molecular sieve membranes. More particularly, the invention pertains to methods of making and using these microporous AlPO4 molecular sieve membranes.
Gas separation processes with membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The design of new materials and efficient methods will further advance membrane gas separation processes within the next decade.
The gas transport properties of many glassy and rubbery polymers have been measured as part of the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, an important limitation in the development of new membranes for gas separation applications is a well-known trade-off between permeability and selectivity of polymers. By comparing the data of hundreds of different polymers, Robeson demonstrated that selectivity and permeability seem to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.
Despite concentrated efforts to tailor polymer structure to improve the separation properties of polymer membranes; current polymeric membrane materials have seemingly reached a limit in the trade-off between productivity and selectivity. For example, many polyimide and polyetherimide glassy polymers, such as Ultem® 1000 polyetherimide, made by GE Plastics, Pittsfield, Mass., have much higher intrinsic CO2/CH4 selectivities (αCOS/CH4) (˜30 at 50° C. and 690 kPa (100 psig) pure gas tests) than that of cellulose acetate (˜22), which are more attractive for practical gas separation applications. These polymers, however, do not have levels of permeability attractive for commercialization compared to current commercial cellulose acetate membrane products, in agreement with the trade-off relationship reported by Robeson. In addition, gas separation processes based on glassy polymer membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer represented by the membrane structure swelling and a significant increase in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases and therefore decreases selectivity.
Inorganic microporous molecular sieve membranes such as zeolite membranes have the potential for separation of gases under conditions where polymeric membranes cannot be used by taking advantages of their superior thermal and chemical stability, good erosion resistance, and high plasticization resistance to condensable gases.
Microporous molecular sieves are inorganic microporous crystalline materials with pores of a well-defined size ranging from about 0.2 to 2 nm. Zeolites are a subclass of microporous molecular sieves based on an aluminosilicate composition. Non-zeolitic molecular sieves are based on other compositions such as aluminophosphates, silicoaluminophosphates, and silica. Molecular sieves of different chemical compositions can have the same or different framework structures. Representative examples of microporous molecular sieves are small-pore molecular sieves such as SAPO-34, Si-DDR, UZM-9, AlPO-14, AlPO-34, AlPO-17, SSZ-62, SSZ-13, AlPO-18, LTA, UZM-25, ERS-12, CDS-1, MCM-65, MCM-47, 4A, 5A, UZM-5, UZM-9, AlPO-34, SAPO-44, SAPO-47, SAPO-17, CVX-7, SAPO-35, SAPO-56, AlPO-52, SAPO-43, medium-pore molecular sieves such as silicalite-1, and large-pore molecular sieves such as NaX, NaY, and CaY. Membranes made from these microporous molecular sieve materials provide separation properties mainly based on molecular sieving and/or competitive adsorption mechanism. Separation with microporous molecular sieve membranes is mainly based on competitive adsorption when the pores of large- and medium-pore microporous molecular sieves are much larger than the molecules to be separated. Separation with microporous molecular sieve membranes is mainly based on molecular sieving or both molecular sieving and competitive adsorption when the pores are smaller or similar to one molecule but are larger than other molecules in a mixture to be separated.
A majority of inorganic microporous molecular sieve membranes supported on porous membrane support reported to date are made from MFI, LTA, FAU or MOR. LTA zeolites have pores in the range of 0.3-0.5 nm, and are able to distinguish small molecules such as H2 and N2. Guan et al. reported a H2/N2 ideal separation factor of 7.1 for a Na−-type LTA zeolite membrane and improved the value to 7.5 by ion-exchange with K− (see Guan et al., SEPARATION SCIENCE AND TECHNOLOGY, 2001, 36, 2233). The pores of MFI zeolites are approximately 0.5-0.6 nm, and are larger than CO2, CH4, and N2. Lovallo et al. obtained a selectivity of about 10 for CO2/CH4 separation using a high-silica MFI membrane at 393° K (see Lovallo et al., AICHE JOURNAL, 1998, 44, 1903). The pores of FAU zeolite are approximately 0.78 nm in size, and are larger than the molecular sizes of H2 and N2. High separation factors have been reported for CO2/N2 mixtures using FAU-type zeolite membranes. Permeation and adsorption experiments indicate that the high separation factors can be explained by competitive adsorption of CO2 and N2.
In recent years, some small-pore microporous molecular sieve membranes such as zeolite T (0.41 nm pore diameter), DDR (0.36×0.44 nm), and SAPO-34 (0.38 nm) have been prepared. These membranes possess pores that are similar in size to CH4 but larger than CO2 and have high CO2/CH4 selectivities due to a combination of differences in diffusivity and competitive adsorption. For example, a DDR type zeolite membrane has shown much higher CO2 permeability and CO2/CH4 selectivity compared to polymer membranes. See Tomita et al., Microporous and Mesoporous Materials, 2004, 68, 71; Nakayama, US 2004/0173094. SAPO-34 molecular sieve membranes showed improved selectivity for separation of certain gas mixtures, including mixtures of CO2 and CH4. See Li et al., ADVANCED MATERIALS, 2006, 18, 2601; Falconer et al., US 2005/0204916.
There remains a need for improved molecular sieve membranes that provide improved selectivity for separations. Previous to the present invention, pure microporous aluminophosphate (AlPO4) molecular sieve membranes such as AlPO-14 and AlPO-18 membranes have not been reported. The present invention discloses novel microporous aluminophosphate (AlPO4) molecular sieve membranes and methods for making and using the same.